This invention relates to means for controlling the path length of a ring laser gyro, and has particular relation to such means which are as completely digital as possible.
In a ring laser gyro (RLG), an optical ring is formed, and two laser beams are directed around the ring in opposite directions. When the beams are combined, rotation of the ring appears as an interference shift in the combined beams.
It is apparent that precise control must be maintained over the optical length of the path which the beams take around the ring. The conventional method is analog dithering. Referring now to FIG. 1, the intensity of the light produced by a laser is schematically plotted as a function of the path length of the RLG. At point 10, the path length is too short for the laser to produce its maximum output; at point 12 it is too long; and at point 14 it is just right.
The path length of an RLG can easily be controlled by controlling the position of one (or more) of the mirrors which bounce the laser beams around the ring. This may most conveniently be accomplished by placing a piezoelectric transducer (PZT) on the back of the mirror, and controlling the thickness of the PZT by controlling the voltage which is fed to the PZT. If the RLG is operating at point 10, then the voltage to the PZT is increased; if the RLG is operating at point 12, the voltage is decreased; and if the RLG is operating at point 14, the voltage is kept constant. The PZT is constructed such that increasing the voltage makes the PZT thinner, which (since the PZT is on the back of the mirror) increases the path length. Decreasing the voltage decreases the path length. Voltage polarity, PZT position, and PZT operation (increased voltage makes it thicker) may be reversed in pairs if convenient.
Dithering is used to determine the point at which the RLG is operating. Dithering is the application of a small AC sinusoidal voltage signal to the PZT, causing the path length of the RLG to likewise vary sinusoidally. Turning now to FIG. 2, the operation of dithering is shown. If the RLG was operating at point 10 without dithering, then it will operate at point 16 and 18, and at every point in between, with dithering. If the RLG was operating at point 12 without dithering, it will instead operate between points 20 and 22; and, if previously operating at point 14, it will now operate between points 24 and 26.
Turning now to FIGS. 3a-3d, the effects of dithering are shown. As is seen in FIG. 3a, the dithering voltage (and consequent PZT position) begins with its average value at point 28, increases to its maximum at point 30, returns to its average value at point 32, falls to its minimum at point 34, and again returns to its average value at point 36.
FIG. 3b shows the concurrent fluctuations of the laser beam's intensity if the RLG is operating at point 10, that is, if the path length is under the length required for peak intensity. Increasing the dithering voltage to point 30 increases the under peak intensity to point 18, and decreasing the dithering voltage through the neutral point 32 to the minimum point 34 causes the under peak intensity to drop from its maximum 18 back through neutral point 10 and to minimum point 16. The voltage and the under peak intensity are in phase.
Conversely, the over peak intensity is out of phase with the dithering voltage, as is shown in FIG. 3c. Increasing the voltage (and PZT position) from point 28 to point 30 causes the over peak intensity to drop from operating point 12 to operating point 22, and decreasing it through point 32 to point 34 causes the intensity to rise through point 12 to point 20. See FIG. 2.
If the RLG is operating at its peak, rather than either under or over its peak, then moving it from operating point 14 to either operating point 26 (voltage point 30) or operating point 24 (voltage point 34) will cause the peak intensity to drop from its maximum to its minimum. See FIG. 2 and FIG. 3d. Instead of being either in phase or out of phase with the dithering voltage, the peak intensity modulates with a frequency twice that of the dithering voltage.
Because the gain curve shown in FIGS. 1 and 2 is relatively flat at the peak 14, in comparison with the sides 10 and 12, the amplitude of modulation of the light intensity is less when the RLG is operating at its peak, in comparison to when it is operating with a path length which is either under or over the peak path length. The modulations at peak are therefore more difficult to detect.
The prior art uses analog components to form a phase comparator, into which is fed the dithering voltage and a pick-off voltage, that is, the voltage produced by a photodetector actuated by a small sample of light which has been picked off from the laser beam. If the two signals are in phase, then the voltage to the PZT is increased; if they are out of phase, it is decreased.
An analog phase comparator and voltage feedback device suffers from the drawbacks of analog devices generally: radiation softness, bulk, weight, lack of tolerance for variation in component parameters, and the like.
It is an objective of the present invention to minimize the use of analog components.
It is a feature of the present invention that it uses analog components only when interfacing with the laser beam, i.e., at the PZT and around the photodetector. The output from the photodetector is first amplified to a useful level, is then band-pass filtered to eliminate both the inevitable noise which is present at frequencies higher than the highest frequency of interest (the modulation frequency when the RLG is operating at its peak [see FIG. 3, bottom line]) and any dc component, and is then immediately fed into a specially designed analog-to-digital converter (ADC). Likewise, the PZT is driven directly by a digital-to-analog converter (DAC).
It is an advantage of the present invention that signal processing takes place between the ADC and the DAC and is entirely digital.